Automated assembly operations, especially those incorporating robots in the assembly process, are widely used in many manufacturing applications. The assembly of computer circuit boards, which are populated with a wide variety of electronic components, is one such example of an assembly process which employs the use of robots. During the manufacture of circuit boards, electronic components are typically picked up by a robot at one stage, and moved to a different stage of the assembly process for the purpose of integrating those components into a higher level assembly. One fairly common method of picking up a component is to have a robotic arm fitted with a vacuum nozzle, also known as an "end effector", lowered into physical contact with the component, at which time a vacuum force causes the component to adhere to the nozzle tip. The component can then be picked up and moved by the robotic arm to the next stage for further processing.
In many such automated operations the end effector is designed so that it is fixed and does not move in any way, other than the movement imparted by the robotic arm (ie. the up and down motion as a component is picked up, for example). Such unmovable end effectors are normally fixed into position such that the plane formed by the pick up surface of the end effector is aligned with the plane formed by the pick up surface of the component's packaging. Accordingly, when the end effector is lowered onto the component a vacuum seal can be created by the aligned planar contact of the end effector with the component. Although such automated assembly processes are designed for this planar alignment of the end effector and the components, sometimes a misalignment occurs such that the plane formed by the contact surface of the end effector and the plane formed by the pick up surface of the component may be at different angles. This differing alignment of the planes formed by these surfaces shall hereinafter be referred to as "unaligned" or "non-aligned" planes or surfaces.
There are a number of factors which may cause these surfaces to be unaligned. For example, many electronic components which generate a considerable amount of heat during operation have heat sinks attached to them for the purpose of heat dissipation. During the manufacturing of the component a thermally conductive glue is placed between the packaging of the electronic component and the heat sink, so that the heat sink adheres to the package. Due to tolerances in this manufacturing process there may be an uneven distribution of glue between the package and the component resulting in the plane formed by the surface of the heat sink and the plane formed by the surface of the packaging of the electronic component being unaligned. Given that during the automated assembly process such components are typically picked up by the heat sink, non-alignment between the heat sink and the component packaging usually translates to non-alignment between the end effector and the heat sink.
Other factors that may contribute to the non-alignment of the surface of the component that is being picked up and the end effector could be the non-alignment of the surface of the component itself, which could thereby cause the heat sink to be unaligned even if the glue is distributed evenly. Or, non-alignment may result from the way the component is positioned in the carrier which carries the component under the robot on the assembly table, and the non-alignment of the carrier, itself. All of these factors, especially when taken in combination, can cause the non-alignment of the pick up surface of an unmovable end effector with the surface of the component which it is intended to pick up.
The consequence of this non-alignment of surfaces is that an end effector which is unmovable is more likely to miss picking up the component when lowered onto the component because the end effector is unable to make an adequate vacuum seal with the pick up surface of the component. The non-alignment causes the vacuum to escape as the tip of the end effector fails to make a flush contact with the component. Accordingly, the productivity of the automated assembly process is impaired as the components do not get processed as intended.
A general solution to this unalignment problem is to design the end effector so that it can adjust to and align with the planar surface of the component that is being picked up. One known approach to aligning a vacuum end effector to the component is shown in U.S. Pat. No. 4,600,228, issued to Tarbuck on Jul. 15, 1986. This patent covers a lockable compliant end effector apparatus useful with a robotic arm for the automated assembly of electronic equipment. The apparatus includes a main body portion which has a socket formed in one end in which a spherical member is received substantially therein and is mounted for pivoting movement. The socket and the spherical member have roughened surfaces in proximity to a fluid passageway which is formed through both such members. A retainer member is removably connected to the end of the main body portion in such a way that the spherical member is retained in the socket, but is capable of pivotally moving. The pivoting movement of the spherical member within the socket enables the end effector to align with the surface of the component to be picked up.
The apparatus shown in Tarbuck further involves the use of an air bearing which enables the spherical member to float in order to align with the component to be picked up. In order to create this air bearing the apparatus first uses a positive air flow, which forces the spherical member down and away from the socket. When the spherical member is lowered onto the component that is being picked up, the spherical member is forced into contact with the socket. A sensor which can determine air pressure build up causes the downward motion of the robotic arm to stop, at which point the direction of the air pressure reverses from a positive direction to a negative direction. Once the negative air pressure, or vacuum, is applied, the roughened surfaces of the main body and the socket are kept in contact with each other, thereby locking the end effector and component in place.
The invention shown in Tarbuck also involves the use of a flow adjustment member which is rotatably mounted in a bore which extends through the spherical member. The flow adjustment member is alignably adjusted relative to the passageway through which the positive and the negative air pressure is applied.
While Tarbuck shows the use of a socket and spherical member for the purpose of aligning the tip of an end effector with a component, a need exists for a simpler device which is easy to make and does not require the use of positive and negative air pressure or the use of refined adjustments to the end effector apparatus.